1. Field of the Invention
The present invention relates to a system for maintaining two copies of data at two different locations.
2. Description of the Related Art
A key feature of any disaster recovery system is the backup of critical data and updates to such backed-up data as soon as possible. The simplest method of backing up data is with off-site backups. However, the extent of successful data recovery in the event of failure is dependent upon the frequency of updates to the backed-up data. If a failure occurs between updates, then data updated but not backed-up may be lost. Thus, the greater time delay between back-ups, the less data is recoverable. On the other hand, increasing the frequency of back-ups causes delays in responding to requests to the volumes being backed-up. Volumes involved in back-up operations are typically unavailable for use by application programs. Thus, increasing the frequency of updates to the data back-up increases the time during which particular volumes involved in the back-up operations are unavailable.
One data back-up system is the Peer-to-Peer Remote Copy (PPRC) function offered by International Business Machines, Corporation ("IBM"), the assignee of the subject patent application. PPRC provides synchronous copying of data from a primary controller to a remote (secondary) controller. The PPRC system includes two controllers and two direct access storage devices (DASDs), a primary controller and associated primary DASD at one site and a secondary controller and secondary DASD at another site. Often, both controllers include a non-volatile storage unit ("NVS"), such as a battery backed up storage unit, to maintain data in the event of a power or system failure. With PPRC, when data is written to the primary NVS, the data is then transferred to the NVS in the secondary controller. At later points in time, the data in the primary and secondary NVSs will be destaged to the attached storage devices. With PPRC, there is minimal risk of losing data upon the occurrence of a failure because data is backed up at the secondary volumes synchronously with updates to the data at the primary DASD managed by the primary controller. The PPRC feature and other backup copy operations are further described in IBM publication "3990/9390 Storage Control Introduction," document no. GA32-0098-08 (IBM Copyright 1987, 1996), which publication is incorporated herein by reference in its entirety.
One problem with prior art PPRC type back-up systems is that the tracks or volumes being backed up between the primary and secondary DASDs are unavailable for use while PPRC back-ups are being serviced. The PPRC operation does not consider the transfer of data to the secondary site complete until all the data updated at the primary DASD has been copied and backed-up to the secondary DASD. Thus, updates to the primary DASD delay response time to user requests to the volumes involved in the update because a synchronous update must be made to the secondary DASD before the volume involved is available. Response delays can occur with respect to requests to both the primary and secondary DASDs. Users making requests to a volume in either the primary or secondary DASD subject to a back-up operation must wait until the completion of the back-up before they can access the updated data.
As the data transfer size and distance between the primary and secondary DASDs increase, the service time of synchronous PPRC updates to both the primary and secondary DASDs increases. Increase in service time for data back-ups resulting from increased distances or larger data transfer sizes likewise delays the response to requests from host systems or application programs for volumes involved in PPRC updates. Moreover, the response time will also increase with respect to requests to volumes in the primary DASD not involved in PPRC updates to the secondary controller. This degradation in response time is often substantial, ranging anywhere from 10% to 200%.